The invention concerns a power-splitting transmission with both a hydraulic and a mechanical power branch.
Hydraulic-mechanical power-splitting transmissions possess generally, first a hydraulic power branch in order, in one area of driving, to be able to change the gear ratio in a stepless manner, and second, a mechanical power branch, which allows shifting through various changes of gear. In the hydraulic power branch, hydrostatic pumps are advantageously employed, such as, for example, axial piston-pumps and/or constant volume pumps, which are adjustable in their delivered volumes. Adjustable hydrostatic motors, such as axial piston motors also find use.
DE 195 22 833 discloses a power-splitting transmission in which, with the mechanical power branch, shifting is possible through four forward (or reverse) gear ranges, and with the hydraulic power branch the speed of rotation, within these gear ranges, can be steplessly varied whereby, in an integrating transmission, both power drives are again combined. In the case of the mechanical power branch, different gear stages are arrived at with the help of friction clutches which are activated by hydraulic activation apparatuses. In order to start these activation apparatuses, a source of pressure (not shown) is required. The hydrostatic power branch comprises a volume-adjustable, hydrostatic pump which is driven by a hydrostatic motor. In order to direct delivery of the hydrostatic pump in both directions, the only possibility which exists is to operate the pump and its hydrostatic motor in a closed circuit, in other words, the output and intake openings of the hydrostatic pump are connected. The leakages, which are lost from this closed circuit, must be compensated for by another make-up pump.
DE 197 06 114 A1 proposes an apparatus for minimizing the pulsations of a hydrostatic positive displacement pump wherein, in the said positive displacement pump, a reservoir element is provided which damps the pulsations. Reservoir elements which, for example, are constructed as hydro-pneumatic tanks, can suffer wear in operation and then must be replaced.
Thus, the purpose of the present invention is to create a power-splitting transmission, with a hydraulic power branch and a mechanical power branch, which possesses a minimum number of components and is characterized by high efficiency.
In accord with the invention, the number of the components is reduced in that the power-splitting transmission employs only one source of pressure, in order to compensate for any leakage in the closed hydraulic power circuit and to supply the activation apparatuses of the shifting elements of the mechanical branch with pressure. The electrohydraulic regulation for the control of the activator apparatuses of the shifting elements of the mechanical power branch react sensitively to pressure variations and pressure pulsations. The hydraulic power branch, using positive displacement piston units, generates pressure pulsations. The said compensation is effected by one source of pressure for both pressure consuming units and having one damping apparatus available between the pressure source and the hydraulic power branch. It is advantageous to place this damping apparatus proximate to the pressure source, that is, the hydraulic power branch. Another possibility is to locate the damping apparatus at any other place in the transmission. If the damping apparatus is installed directly near the pulsation producer, that is, in the housing of the hydrostatic positive pressure unit, then an optimal action of the damping apparatus is brought about. In such case, the hydrostatic positive pressure unit can be placed with already damped pulsations in the housing of the power-splitting transmission. The pulsation damping apparatus preferably comprises a space which allows an expansion of the medium volume and thus diminishes the pulsations. Since this damping apparatus consists exclusively of an expansion of the volume in the line between the source of pressure and the hydraulic power branch, no components exist in the damping apparatus which can suffer abrasion and must be replaced. The space, which allows volume expansion, can be also cast in the housing of the power-splitting transmission whereby an additional reduction of the number of individual parts is gained. Additionally, the line from the source of pressure can also be cast in the housing of the power-splitting transmission. However, it is still possible that the line from the source of pressure to the hydraulic power branch can be made as a pipe line and the damping arrangement can be designed as a pipe section of a larger volume.
Since only one source of pressure first serves the activation apparatuses of the shifting elements of the mechanical power branch through a control unit and second, compensates for the leakage in the hydraulic power branch, the number of parts in the power-splitting transmission are reduced and the efficiency is increased.